Free Duct Sizing Calculator
Calculate the required duct diameter for a given airflow and maximum velocity using the equal friction method.
CFM
ft/min
Round Duct Diameter
10.2 in
Cross-Section Area82.3 sq in
Actual Velocity at Next Size606 ft/min
Round Duct Diameter vs Airflow
Duct Sizing by Velocity
A = CFM / Velocity
Diameter = 2 x sqrt(A / pi)
Convert the area to diameter for round ducts. For residential trunk lines, maximum velocity of 700 ft/min keeps noise low. Branch runs can go up to 600 ft/min. Round up to the next standard duct size.
Example Calculation
400 CFM at a max velocity of 700 ft/min.
- 01Area = 400 / 700 = 0.571 sq ft = 82.3 sq in
- 02Diameter = 2 x sqrt(0.571 / pi) x 12 = 10.2 in
- 03Use a 10-inch round duct (or 12-inch for lower velocity).
Frequently Asked Questions
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